Difference Between Sidereal Day and Solar Day

In general, a day is considered as the time taken by earth to complete one revolution around its axis. This concept has been the basis of time measurement for the majority of the human history. The day can be further divided into smaller time units, and time can be measured by the angle made by the sun during two events.

Later with the development of astronomy, the concept of sidereal day and sidereal time were introduced.

Solar Day

The time between two successive passes across the meridian by the sun is known as a solar day. The time measured by this method (by observing the position of the sun in the sky) is known as solar time. The mean solar day is about 24 hours, but varies based on the position of the earth in its orbit relative to the sun. The length of mean solar day is increasing due to the tidal acceleration of the moon by the earth and corresponding deceleration of the earth`s rotation.

Sidereal Day

Sidereal day is measured based on the motion of the earth relative to the “fixed” stars in the sky. Technically, a sidereal day is the time between two successive upper meridian passages of the vernal equinox.

Because of the rotation of the earth around the sun and its axis, earth makes one rotation and moves approximately 1^0 along the orbit. This movement causes a lack of 4 minutes in a single rotation. Therefore, the sidereal day is 23h 56m 4.091s

What is the difference between Sidereal Day and Solar Day?

• The sidereal day is based on the successive passes of meridian across the vernal equinox, while solar day is a measure based on the successive passes of the sun.